1. Field of the Invention
This invention relates to torque measurement in a mechanical device by sensing axial shaft movement and in particular the torque output of a grear speed reducer.
2. Description of the Prior Art
All rotating machines such as motors, generators, turbines and mechanical gear trains or transmissions have one major function in common; they produce or transmit torque. They all also absorb some torque which becomes a power loss to the overall system. The method and apparatus for sensing and measuring this torque output power and loss have been a continuous challenge to the engineers in the related field. Most methods for measuring the torque require a modification of the drive train at some point and a device inserted into it or fastened on it to measure torque, such as that described in U.S. Pat. No. 2,359,125. The device described in that patent utilizes strain gages for both the thrust and torque measurement. Another method of measuring torque shown in U.S. Pat. No. 3,595,074 consists of a pair of axially spaced plates having a plurality of flexure plates mounted therebetween wherein one plate is attached to a rotary machine housing and the other plate to the drive mechanism. The drive mechanism is supported by this torque transducer from the housing surface and is mounted in cantilever fashion at the unsupported ends of the flexure plates.
The reaction or countertorque between the mounting plates, in proportion to the input or output torque, and the relative movement of one plate with respect to the other plate, is measured and hence the torque determined by use of a differential transformer, which develops an electrical signal proportional to the angle through which the end plate is rotated.
In still another U.S. Pat. No. 3,745,819, the axial thrust developed by tapered roller bearings in response to radial loads exerted on the rotating member is fed through a piston adapted to move axially against a pressure chamber to provide a pressure reading proportional to the radial load imposed on the rotatable member. This device required an additional pressurized and sealed mechanism inside the housing or attached directly to it which has many components in it and is very expensive to manufacture. Other U.S. patents, such as U.S. Pat. No. 2,957,343 shows still another method for measuring torque as a function of thrust using air pressure and in U.S. Pat. No. 1,998,450 the thrust is measured by deflecting a spring which move the plates of an electrical condenser causing a change in the capacitance which is proportional to the axial movement of a shaft. Still other methods employ slip rings and require cutting the shaft and insertion of the transducer into the shaft member itself to measure the torque of that shaft. Still another method commercially available employs a Linear Variable Differential Transformer (LVDT) wherein the core position moves as a function of the shaft thrust and hence torque output of the device. Such a unit, however, is very large and requires an extensive modification of the power device. My invention gives the same net result as an LVDT but with a much simpler and more compact package. As discussed, many methods have been used to detect the rotary motion or axial thrust of a shaft and translate this to the torque developed by the shaft. Although some of the aforementioned patents and methods were used in conjunction with gear trains and power transmissions none of these solved the problem in the unique way as does my invention.
A gear box and, for that matter, other power transmission systems, usually operate with a continuous bath of oil available to the gears when in use. This lubrication is necessary for translating the high speed and low torque input into a low speed and high torque output. Such units are designed to be as small and compact as possible. A device to measure output torque therefore, must also be small and compact in size to fit into an existing housing or space, and be low in cost and capable of constant immersion in oil or other fluids, operate at varying temperatures and contaminated environments and have a method of remote readout. This remote requirement is necessary in many applications of power transmission and gear reducers since a visual inspection during operation of the assembly itself is not possible because of its location. Also in gear devices as well as other machinery, bearings and shafts must maintain their position very accurately under all conditions to assure proper load of the power transmitting parts. Deflections and movements must be minimized often to less than 0.001 inches.